The present application relates to a display apparatus having a polarization control element capable of performing polarization control using a control signal, between a light source and a display panel.
Conventionally, a liquid crystal display apparatus is used as a display apparatus of a personal computer, a television, a cellular phone, or the like. Generally, a liquid crystal display apparatus has a structure in which a liquid crystal display panel obtained by sandwiching a liquid crystal cell by a pair of transparent electrodes is sandwiched by a pair of polarizers whose transmission axes are orthogonal to each other. By changing the angles of liquid crystal molecules in the liquid crystal cells and adjusting the light amount of a backlight, an image is displayed.
In a liquid crystal display apparatus, it is extremely important to make contrast high in order to increase the commercial value. At present, as each of the polarizers provided on both sides of the liquid crystal display panel, a polarizer having a brightness transmittance of polarized light parallel to the transmission axis, of about 75% to 85%, and a brightness transmittance of polarized light orthogonal to the transmission axis, of about 0.005% or less, is generally used. A high-grade model in which the brightness transmittance is 0.001% or less is in the market. In the case where polarizers having such brightness transmittance are provided on both sides of a liquid crystal display panel, the contrast is ideally equal to brightness transmittance ratio and is supposed to be at least 20,000 to 30,000:1 or higher.
However, in reality, due to the influence of light scattering in liquid crystal cells, scattering of pigment particles in a color filter and the like, and birefringence of a phase difference film, slight depolarization of polarized light incident on the liquid crystal display panel occurs. Consequently, the polarization direction of the polarized light incident on the liquid crystal display panel is disturbed, and the transmittance of entire cells in black display becomes higher than that of two polarizers overlapped so that the transmission axes are orthogonal to each other. As a result, the contrast becomes about 1000:1 to 3000:1 which is a value far from the polarization degree of the polarizers.
As a method of improving contrast, a measure using local turn-on/off of a light source is proposed. In the measure, an output of the light source is adjusted according to lightness of a video image, and contrast may be largely improved as compared with a method of increasing the polarization degree of the polarizers or reducing depolarization of the liquid crystal cells. In particular, in the case of a device in which a number of LEDs are arranged just below a liquid crystal display panel is used as a backlight, since partial light-on or light-off is easily performed and response is high, the contrast may be easily improved by using the method.
However, the method may not be used in the case where the light source is a cold cathode tube or is of an edge light type using a light guide plate. In the former case, on/off control may not be performed in a direction parallel to the extension direction of the cold cathode tube, and response is lower than that of an LED at light emission speed of a general phosphor. In the latter case, the light source may not be locally turned on because of the structure. Consequently, the light sources of those types are also requested to largely improve the contrast.
As methods of improving contrast regardless of the type of the light source, for example, Japanese Unexamined Patent Application Publication Nos. 2008-51912, 2008-15289, H03-055592, and H03-113427 disclose techniques preparing another display apparatus in which a liquid crystal display panel is sandwiched by a pair of polarizers whose transmission axes are orthogonal to each other and using the apparatus as an apparatus for an optical shutter.